Thermodynamic Properties of Superconducting and Non-Superconducting Pr 2 Ba 4 Cu 7 O 15−δ Compounds with Metallic Double Chains
Takahisa Konno, Michiaki Matsukawa, Keisuke Sugawara, Haruka Taniguchi
aa r X i v : . [ c ond - m a t . s up r- c on ] D ec Thermodynamic Properties of Superconducting and Non-SuperconductingPr Ba Cu O − δ Compounds with Metallic Double Chains
Takahisa Konno, Michiaki Matsukawa, ∗ Keisuke Sugawara, Haruka Taniguchi, Junichi Echigoya, Akiyuki Matsushita, Makoto Hagiwara, Kazuhiro Sano, Yoshiaki Ohno, Yuh Yamada, Takahiko Sasaki, and Yuichiro Hayasaka Department of Materials Science and Engineering, Iwate University, Morioka 020-8551, Japan National Institute for Materials Science, Ibaraki 305-0047 Kyoto Institute of Technology, Kyoto 606-8585,Japan Department of Physics Engineering, Mie University, Tsu 514-8507,Japan Department of Physics, Niigata University, Niigata 950-2181,Japan Institute for Materials Research, Tohoku University, Sendai 980-8577,Japan (Dated: July 12, 2018)To examine the thermodynamic properties of Pr Ba Cu O − δ compounds with metallic CuOdouble chains, we measured the specific heats of superconducting and non-superconducting poly-crystalline samples at low temperatures (1.8-40 K) under various magnetic fields (up to 9 T). In theas-sintered non-superconducting sample, a λ − like enhancement in the specific heat measurementappeared near the antiferromagnetic transition temperature T N = 17 K. In contrast, the reducedsuperconducting sample with T c,on = 26 . ions under the crystal field effect. PACS numbers: 74.25.Ha,74.25.F-,74.90.+n
I. INTRODUCTION
Since the discovery of high- T c copper-oxide supercon-ductors, strongly correlated electron systems have beenextensively investigated. Besides the physical propertiesof two-dimensional CuO planes, researches have focusedon the physical role of one-dimensional (1D) CuO chainsin some families of high- T c copper oxides.Structurally, the Pr-based cuprates, PrBa Cu O − δ (Pr123) and PrBa Cu O (Pr124), are identical totheir corresponding Y-based high- T c superconductors,YBa Cu O − δ (Y123) and YBa Cu O (Y124). Pr123and Pr124 compounds have insulating CuO planes andare non-superconductive. The suppression of supercon-ductivity in the Pr substitutes has been explained by thehybridization of Pr-4 f and O-2 p orbitals. The crystalstructure of Pr124 with CuO double chains differs fromthat of Pr123 with CuO single chains. It is well knownthat CuO single chains in Pr123 and CuO double chainsin Pr124 show semiconducting and metallic behaviors,respectively. The carrier concentration of doped doublechains of Pr124 is difficult to vary, because it is thermallystable up to high temperatures. In addition, Pr ions inboth Pr123 and Pr124 become antiferromagnetic orderedat the antiferromagnetic transition temperature T N = 17K. The compound Pr Ba Cu O − δ (Pr247) is an inter-mediate between Pr123 and Pr124. In this compound,CuO single-chain and double-chain blocks are alternatelystacked along the c -axis (see Fig.1). The physical prop-erties of the metallic CuO double chains can be exam-ined by controlling the oxygen content along the semi-conducting CuO single chains. Anisotropic resistivity measurements of single-crystal Pr124 have revealed thatmetallic transport arises by the conduction along theCuO double chains. In oxygen removed polycrystallinePr Ba Cu O − δ , superconductivity appears at an onsettemperature T onc of ∼
15 K. Hall coefficient measure-ments of superconducting Pr247 with T onc = 15 K haverevealed that at intermediate temperatures below 120 K,the main carriers change from holes to electrons, as thetemperature decreases. Accordingly, this compound is anelectron-doped superconductor. In our previous study,we examined the effect of magnetic fields on the super-conducting phase of Pr247. Despite of the resistive dropassociated with the superconducting transition, we foundthat the diamagnetic signal was strongly suppressed asexpected in the 1D superconductivity of CuO doublechains. We also reported the temperature dependenceof the Hall coefficient in superconducting Pr247 with ahigher T onc ∼
27 K. Our findings indicated that the su-perconducting transition temperature increased becausethe density of doped electron carriers became denser un-der the reduction treatment, consistent with a theoreticalprediction. In this paper, we demonstrate the thermodynamicproperties of the electron-doped metallic double-chaincompound Pr Ba Cu O − δ , which has a higher T onc (26.5 K), under different magnetic fields. Figure 1 dis-plays the typical crystal structure of Pr Ba Cu O − δ ,in which the CuO metallic double chains and semicon-ducting single chains are alternately stacked along the c -axis. Section II outlines the experimental methods, andSec. III presents the magneto-transport and magneto-thermodynamic properties of polycrystalline Pr247 sam-ples. Along with the magneto-transport data, we reportthe temperature dependences of the specific heats in theas-sintered and reduced Pr247. These results are dis-cussed in terms of a three-level system (quasi triplet),in which the ground-state of the Pr ions is split bythe crystal field effect. The final section is devoted to asummary. Fig. 1: (color online) (a)Typical crystal structure ofPr Ba Cu O − δ compound. Blocks of CuO single chains(denoted S ) and CuO double chains ( D ) are alternatelystacked along c -axis. (b) TEM image of superconductingPr Ba Cu O − δ compound with a superconducting onsettemperature T onc of 26.5 K. II. EXPERIMENT
Polycrystalline samples of Pr Ba Cu O − δ (Pr247)were synthesized by the citrate pyrolysis method. Af-ter several annealing processes, the resulting precursorswere pressed into a pellet and calcined at 875-887 ◦ Cfor an extended period over 120-180 h under ambientoxygen pressure. The oxygen in the as-sintered samplewas removed by reduction treatment in a vacuum at 500 ◦ C for 48 h, yielding a superconducting material. Typi-cal dimensions of the pelletized rectangular sample were9 . × . × . . The Pr247 sample with T onc = 26 .
5K was observed by high-resolution transmission electronmicroscopy using a JEOL3010 microscope operated at 300 kV at Tohoku University. The local crystal struc-ture was analyzed from images obtained by the high-angle annular dark-field scanning transmission electronmicroscope method. The oxygen deficiency in the sam-ple with T onc = 26 . δ = 0 .
56 from gravimetric anal-ysis. As a function of the oxygen deficiency, the T onc rises rapidly at δ ≥∼ .
2, then monotonically increaseswith increasing δ , and finally saturates around 26-27 Kat δ ≥∼ . Accordingly, the carriers in the presentsample are concentrated around the optimally doped re-gion.The electric resistivity in zero magnetic field was mea-sured by the dc four-terminal method. The magneto-transport up to 9 T was measured by the ac four-probe method using a physical property measuring sys-tem (PPMS, Quantum Design) , increasing the zero-field-cooling (ZFC) temperatures from 2 K to 40 K. Thehigh field resistivity (up to 14 T) was measured in a su-perconducting magnet at the High Field Laboratory forSuperconducting Materials, Institute for Materials Re-search, Tohoku University. The electric current I wasapplied longitudinally to the sample ; consequently, theapplied magnetic field H was transverse to the sample(because H ⊥ I ). The specific heats in the ZFC modewere measured to be between 2 K and 40 K by thePPMS. For comparison, the temperature dependence ofthe specific heat of the superconducting Y Ba Cu O − δ (Y247) compound prepared by the high-pressure oxygenmethod was separately measured in zero field at NIMS.The dc magnetization was performed under ZFC ina commercial superconducting quantum interference de-vice magnetometer (Quantum Design, MPMS). Magneticfields of 0.002 T, 0.01 T, 1 T, 3 T, and 5 T were applied. III. RESULTS AND DISCUSSION
Figure 1 shows the typical crystal structure and aTEM image of the superconducting Pr Ba Cu O − δ compound with T onc = 26 . S and D denoteCuO single-chain and double-chain blocks, respectively.We observe a regular stacking structure of { -D-S-D-S-D } . In the x-ray diffraction pattern of polycrystallinePr Ba Cu O − δ synthesized by the citrate pyrolysismethod, a clear peak corresponding to the Miller index(004) of Pr247 was observed, but peaks of Pr123 or Pr124phases were absent (data not shown). Figure 2 plots the temperature dependences of themagnetic susceptibilities of the non-superconducting andsuperconducting samples under various magnetic fields(up to 5 T). In the as-sintered sample, a typical anomalyin χ occurs near 17 K. This anomaly is associated with anantiferromagnetic transition of the Pr sublattice, as pre-viously reported in Pr123 and Pr124 systems. On thecontrary, the 48-h-reduced sample becomes superconduc-tive below T onc = 26 . f was estimated from the magnetic data taken -5 -5 -5 -5 -5 -5 c ( e m u / g O e ) T (K)T N ( a ) as-sintered -5 -5 -5 -5 -5 -5 -5 c ( e m u / g O e ) T (K) ( b ) 48h reduced -4 10 -3 -3 10 -3 -2 10 -3 -1 10 -3 M / H ( e m u / g* O e ) V o l u m e fr ac ti on ( % ) T (K) T C ON = 26.5K
ZFC 10 mT
Fig. 2: (color online) Low-temperature dependences of mag-netic susceptibilities χ of Pr Ba Cu O − δ compounds un-der various magnetic fields (1 T, 3 T, and 5 T). (a)Non-superconducting as-sintered sample and (b) superconducting48-h-reduced sample with T c,on = 26 . T onc = 26 . at 10 mT by f = χ · ρ/ π × χ and ρ denotethe magnetic susceptibility per unit weight and the sam-ple density (5.3 g/cm ), respectively. As displayed in theinset of Fig.2 (b), the volume fraction reaches ∼
30 % at4 K, indicating bulk superconductivity.However, under high fields, the magnetic data of thesuperconducting sample monotonically increases as thetemperature decreases to below 17 K and saturate atlow temperatures. This trend reflects the ferromagneticcharacter of the sample. As highlighted in our previ-ous paper, the diamagnetic signal is suppressed despitethe resistive drop associated with the superconductingtransition. This observation is closely related to the 1Dsuperconductivity of CuO double-chains. In contrast tothe positive magnetization data at higher field, the resis-tivity data of the 48-h-reduced sample substantially dropas the transport currents becomes superconducting (seeFig.3(b)). The clear difference in the magnetic behav- R e s i s ti v it y ( m W c m ) T (K) ( b ) 48h reduced R e s i s ti v it y ( m W c m ) T (K) ( a ) as-sintered r ( m W c m ) T (K) as-sintered48 h reduced
Fig. 3: (color online)Low-temperature dependences of electricresistivities of Pr Ba Cu O − δ compounds measured undervarious magnetic fields. (a) As-sintered non-superconductingsample, measured at 0 T, 3 T, 6 T, and 9 T, and (b)48-h-reduced superconducting sample with T c,on = 26 . T (between2 K and 300 K). . ior between the as-sintered and reduced samples reflectsthe different magnetic interactions among the Pr mag-netic ions. The Pr atoms in Pr124, are well separated,so their antiferromagnetic ordering is not dominated bythe superexchange interaction. Instead, we consider thatthe antiferromagnetic coupling of the Pr sublattice occursdue to the Rudermann-Kittel-Kasuya-Yoshida (RKKY)interaction and is mediated by itinerant carriers as notedby Xu et al. In Pr247, the vacuum reduction treatmentvaries the distance between the Pr ions, and the ferro-magnetic property of the reduced sample occurs throughthe RKKY interaction.From the magnetic susceptibility measurements overa wide range of temperatures range (20-200 K), we es-timate the effective magnetic moment µ eff of the Prions by the Curie-Weiss law. Performing the calcula-tion, we obtained that µ eff =3.02 and 3.26 µ B for thenon-superconducting and superconducting samples, re- C / T ( J / m o l K ) Temperature(K) T N =17K ( a ) as-sintered C / T ( J / m o l K ) Temperature(K) T c,on =27K ( b ) 48h reduced Fig. 4: (color online) Temperature dependences of specificheats of Pr Ba Cu O − δ compounds under various mag-netic fields (up to 9 T). (a) The non-superconducting as-sintered sample and (b) the superconducting 48-h-reducedsample with T c,on = 26 . Ba Cu O − δ (Y247)prepared in high-pressure oxygen are also plotted.. spectively. These values are almost consistent with theeffective moment of Pr124 ( µ eff = 3 . µ B ). In partic-ular, the µ eff of the superconducting sample is close tothat of Pr (3.54 µ B ).The magnetoresistance of the as-sintered and 48-h-reduced samples also differ at high temperatures (above T onc = 26 . C/T of the non-superconducting and su-perconducting Pr247 compounds under several magneticfields (up to 9 T). The
C/T of the non-superconductingas-sintered sample exhibits a λ − like enhancement associ-ated with the antiferromagnetic transition of Pr ions, aspreviously reported in non-superconducting Pr123 andPr124 systems. As the external magnetic field in- C / T ( J / K m o l ) T(K)48h reduced 0T
Fig. 5: (color online) Temperature dependences of specificheat differences ∆
C/T observed in Pr247 and Y247 ( C Pr247 /T and C Y247 /T , respectively). Dashed curves are calculatedSchottky heat anomalies due to crystal field splitting of Pr .Parameters under zero field were fitted as E = 23 . K and E = 46 . K . . creases, this sharp increase is suppressed and peaks atlower temperatures. At 9 T, the thermodynamic peak isconsiderably depressed and is located around 15 K. Atlow temperatures (below 5 K), the specific heat also ex-hibits a strong field dependence, for reasons which havenot yet been clarified. On the other hand, the C/T ofthe superconducting sample is not enhanced at temper-atures above 15 K, regardless of the applied magneticfield. The absence of λ − like enhancement in reducedPr247 appears to be consistent with the correspondingmagnetic behavior, which shows no clear AFM transition.At temperatures below 15 K, the C/T of the supercon-ducting sample shows a broad peak around 8.5 K. Thisbroad anomaly is strongly suppressed under a magneticfield. For comparison, Fig.4 also plots the temperaturedependence of the specific heat of superconducting Y247in the absence of magnetic field, which was preparedby the high pressure oxygen method. Oxygen removedY Ba Cu O , with no superconductivity down to 2 K,exhibits a C/T magnitude and temperature dependencesimilar to those of of the superconductor Y247. Ac-cordingly, because Y247 and Pr247 share the same crys-tal structure, we roughly assumed the low-temperaturespecific heat of superconducting Y247 to be the latticecomponent of the Pr247 sample. We further assumedthat the low-temperature dependences of the electroniccomponents of both Y247 and Pr247 were significantlysmaller than those of the lattice parts.Figure 5 plots the specific heat differences dC/T between the superconducting Pr247 and Y247 data,( C Pr247 /T and C Y247 /T , respectively) versus tempera-ture. Comparing our results with the previous dataobtained using Pr-based 123, it is evident that thelow-temperature specific heat data of PrBa Cu O andPrBa Cu O follow temperature trends, similar to thoseof as-sintered and vacuum-reduced Pr247, respectively.The common features include the λ -like enhancement inthe former compounds and the broad C/T maximum inthe latter. To analyze the low-temperature peak in thespecific heat trends of PrBa Cu O , Hilscher et al in-troduced a Schottky type specific heat contributed by athermal population of electrons. In this study, we fol-lowed Hischer’s approach to understand the broad max-imum in the C/T of the corresponding superconductingPr247. In the Pr123 system, the crystal field groundstate of the Pr ion is a quasitriplet and is separated byseveral tens of meV from the group of remaining crystalfield levels.
Accordingly, we assumed that at tem-peratures below 30 K, the energy levels of the Pr ion aresplit into three by the crystal field effect.Assuming that for three energy levels E i ( i =0 , , and 2) E =0 K and E < E , we then get a par-tition function for the three energy level system, Z =1 + e − βE + e − βE ( β = 1 /kT ). Substituting the aboveformula into a general expression for a specific heat, C = kT d log Zdβ , a Schottky-type specific heat with threeenergy levels is given by C Sch = k B x e − x + y e − y + ( x − y ) e − x − y ( e − x + e − y + 1) where x = E /T and y = E /T denote the temperaturereduced energy levels.As shown in Fig.5, the zero field data below 30 Kare roughly fitted by the Schottky expression with E =23 . K and E = 46 . K . The energy levels fitted to thecurves of the superconducting Pr247 compound are sim-ilar to the experimental and calculated crystal field lev-els of the H multiplet in PrBa Cu O , which has or-thorhombic crystal field parameters. For example, as pre- dicted from inelastic neutron scattering measurements ofPrBa Cu O , the crystal field splitting of this compoundis E = 19 . K and E = 39 . K . The low-temperaturepeak in the specific heat of the oxygen-removed super-conducting sample is reasonably described by the Schot-tky anomaly (see Fig.5). The small discrepancy betweenthe calculated curve and the experiment data is prob-ably attributed to magnetic interactions among the Prions. The reduced superconducting sample shows no ob-vious anomaly associated with the superconducting tran-sition, inconsistent with the experimentally observed re-sistive drop and diamagnetic signal as the temperaturedecreases. As shown in the inset of Fig.2, the supercon-ducting volume fraction is several percent around 15 K,and rapidly increases as the temperature falls below 10K. Thus, we infer that the superconducting anomaly ofmetallic CuO double chains is masked by the Schottkylike broad enhancement in
C/T caused by the crystalfield effect of the Pr ions.
IV. SUMMARY
We demonstrated the thermodynamic proper-ties of superconducting and non-superconductingPr Ba Cu O − δ compounds with metallic CuO doublechains. For this purpose, we measured the specific heatsof the polycrystalline samples at low temperatures undervarying magnetic fields (up to 9 T). The specific heatof the as-sintered non-superconducting sample displayedthe λ − like enhancement near the antiferromagnetic tran-sition temperature T N = 17 K. This anomaly around thesuperconducting transition was absent in the reducedsuperconducting sample for reasons related to the smallsuperconducting volume fraction at temperatures above15 K. As the temperature decreased below 15 K, the C/T of the superconducting sample broadly peakedaround 8.5 K. This Schottky-like broad maximum wasattributed to low-lying quasi-triplet splitting of Pr ions under the crystal field effect. Acknowledgments
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